Published 4:00 am, Sunday, April 13, 2008

Some people know, with absolute certainty, that they were born the wrong gender.

A girl sees that she has no phallus, yet she feels deeply, unambiguously male. A boy is equipped with a penis, yet he feels fundamentally, unarguably female.

Such discord often gets chalked up to the physical - prenatal hormone exposures, abnormal brain structures, gay genes. Or to the psychological - repressed homosexuality, absent dads, overbearing moms, parents who wanted a baby of the opposite sex.

But there is a new explanation: Some transgender men claim to possess phantom penises. From the time they were little girls, they say they had vivid sensations of a penis between their legs. Others develop such a phantom when they begin taking testosterone therapy.

Similarly, transgender women who are born male and later undergo sex reassignment surgery generally do not report having a phantom. They say that their penis was never part of their body image.

V.S. Ramachandran, a neurologist and psychologist at UC San Diego and a leading authority on phantom limb sensations, says it has long been known that some people who are born without arms have vivid phantom arms. They can swing them around, wave goodbye and make complicated gestures.

This suggests that an intact body image - the maps of the body laid down in the brain before and after birth - can develop without actual limbs. So-called mirror neurons that map the actions and intentions of others into one's own brain may help bring the phantoms to life, Ramachandran says.

But phantoms might also exist from the beginning of life. For transgender men and women, he says, the body image laid down prenatally could similarly differ from the external body anatomy.

Simon LeVay, an expert on human sexuality, says that Ramachandran is comparing those who are extremely pleased with getting rid of their penis to others who are distressed and think about their penis all the time. "Emotions are left out," LeVay said. "I am not sure he has looked at the question of wishful thinking in detail."

Phantom limbs were first described as a medical condition after the Civil War. Amputees said then, as they do now, that they continue to experience bodily sensations as if the absent limb were still present. But without any scientific explanation, phantoms were chalked up to wishful thinking. But in the early 1990s, Ramachandran carried out experiments that demystified phantoms. They are not the stuff of human imagination. Rather they are a product of brain wiring.

When a limb is amputated, the area of the brain representing that limb is no longer activated by touch. But such areas do not become vacant lots. They get invaded by nerve fibers from adjacent brain areas that map intact body parts. When those parts - say the face or shoulder - are touched, sensations are felt in the missing limb.

Not long after this discovery, a few people wrote to Ramachandran to say that they experienced phantom penises after losing the organ in an accident or to disease. They even had phantom orgasms.

The first finding was intriguing. Only 6 out of 20, or 30 percent, of the transsexual women who had had their penises removed reported feeling a phantom phallus. But 58 percent of "normal" men have such sensations after the surgery.

The second finding was surprising. A third to a half of "normal" women experience phantom breasts after a mastectomy, as opposed to only 3 out of the 29 transgender men. The third finding was downright astounding. Among the transsexual men, 18 out of 29, or 62 percent, said they had experienced a phantom penis long before their surgery.

In two cases, the phantom appeared shortly after the start of testosterone therapy. "If the phantom is a result of wishful thinking, why would a hormone be required to trigger it?" Ramachandran asks.

The findings imply that transsexuality should not be regarded as abnormal, Ramachandran says. No rigid barriers exist between the sexes. Rather, sexual identity exists along a biological continuum that involves an innate body plan and life experience.

"I expect a lot of criticism," Ramachandran says. "Those who study transsexuality tend to be territorial because they themselves have made so little progress. There is no literature that illuminates the underlying mechanisms, other than psychological mumbo jumbo. And then someone comes striding in and spends two weeks solving the riddle. It must be infuriating."

Latest from the SFGATE homepage:

Click below for the top news from around the Bay Area and beyond. Sign up for our newsletters to be the first to learn about breaking news and more. Go to 'Sign In' and 'Manage Profile' at the top of the page.